Power field effect transistors, e.g., MOSFETs (metal oxide semiconductor field effect transistors), are well-known in the semiconductor industry. One type of power MOSFET is a DMOS (double-diffused metal oxide semiconductor) transistor. A cross-sectional view of a portion of a cell array of one known variety of DMOS transistors is shown in FIG. 1. As shown, an n-type epitaxial layer 102 overlies n-type substrate region 100 to which the drain contact is made. Polysilicon-filled trenches extend into the epitaxial layer 102 from the top surface. The polysilicon 106a, 106b in the trenches are insulated from the epitaxial layer by oxide layers 104a, 104b. Source regions 108a, 108b in p-type body regions 110a, 110b are adjacent the trenches at the top surface. A polysilicon gate 114 overlaps the source regions 108a,b, extends over a surface portion of the body regions 110a,b, and extends over a surface area of a region between the two trenches commonly referred to as the mesa drift region. Metal layer 116 electrically shorts source regions 108a,b to body regions 110a,b and polysilicon 106a,b in the trenches. The surface area of body regions 110a,b directly underneath gate 114 defines the transistor channel region. The area between body regions 110a and 110b under gate 114 is commonly referred to as the JFET region.
Upon applying a positive voltage to the gate and the drain, and grounding the source and the body regions, the channel region is inverted. A current thus starts to flow from the drain to the source through the drift region and the surface channel region.
A maximum forward blocking voltage, hereinafter referred to as “the breakdown voltage”, is determined by the avalanche breakdown voltage of a reverse-biased body-drain junction. The DMOS structure in FIG. 1 has a high breakdown voltage due to the polysilicon-filled trenches. Polysilicon 106a,b cause the depletion layer formed as a result of the reverse-biased body-drain junction to be pushed deeper into the drift region. By increasing the depletion region depth without increasing the electric field, the breakdown voltage is increased without having to resort to reducing the doping concentration in the drift region which would otherwise increase the transistor on-resistance.
A drawback of the FIG. 1 structure is its high output capacitance Coss, making this structure less attractive for high frequency applications such as radio frequency (RF) devices for power amplifiers in the wireless communication base stations. The output capacitance Coss of the FIG. 1 structure is primarily made up of: (i) the capacitance across the oxide between the polysilicon in the trenches and the drift region (i.e., Cox), in series with (ii) the capacitance across the depletion region at the body-drift region junction. Cox is a fixed capacitance while the depletion capacitance is inversely proportional to the body-drain bias.
The breakdown voltage of power MOSFETs is dependent not only upon the cell structure but also on the manner in which the device is terminated at its outer edges. To achieve a high breakdown voltage for the device as a whole, the breakdown voltage at the outer edges must be at least as high as that for the cells. Thus, for any cell structure, a corresponding terminating structure is needed which exhibits a high breakdown voltage.
In most amplifier circuits a significant amount of heat energy is produced in the transistor. Only 50% efficiency is typical of the best class AB RF power amplifiers available. An important factor in designing power devices for high frequency applications is thus the thermal performance of the device. Because of the different device performance requirements, the cells in power MOSFETs are densely packed resulting in concentration of heat in active regions and poor heat transfer rates. The increase in temperature resulting from the poor heat transfer rate adversely effects the device performance.
Thus, a power MOSFET device with such improved characteristics as low output capacitance, high breakdown voltage, and improved thermal performance is desired.